1. Field of the Invention
This invention relates generally to a radar apparatus for detecting a target. More particularly, it relates to an FM radar apparatus for detecting a target on the basis of a beat signal (frequency deviation between transmitted and received signals) obtained by mixing a transmitted signal of FM wave with frequency varying with time, and a received signal of reflected wave from the target.
2. Description of the Related Art
FM radar apparatuses, which transmit and receive frequency modulated (FM) signals and detect a target on the basis of a beat signal obtained by mixing transmitted and received signals, are known from, for example, Japanese Patent Laid-Open Publication No. HEI 5-180931, Japanese Patent Laid-Open Publication No. HEI 8-211145, Japanese Patent Laid-Open Publication No. HEI 7-128440, Japanese Patent Laid-Open Publication No. HEI 6-242230, Japanese Patent Laid-Open Publication No. HEI 4-142486, Japanese Patent Laid-Open Publication No. HEI 5-142337 and Japanese Patent Laid-Open Publication No. HEI 5-150035.
In Japanese Patent Laid-Open Publication Nos. HEI 5-142337 and HEI 5-150035, there are disclosed FM radar apparatuses in which a beat signal is subjected to a frequency analysis using a Fast Fourier transform (FFT) to provide a plurality of peak frequencies, and a distance to and a speed of a target are detected based on the peak frequencies in each of frequency rise and fall regions of a frequency modulated triangular wave (frequency rise and fall domains of FM signal).
In these publications, there are also disclosed an arithmetic operation in which the sum of and difference between the peak frequencies in the rise and fall domains of the beat signal are divided by two to thereby detect the distance and speed. Another arithmetic operation is also disclosed which may be used to detect a relative distance and a relative speed with respect to the target. Herein-below, these arithmetic operations will jointly be called "Doppler shift calculation".
The above two publications teach detection of the distance to and speed of the target from the peak frequencies of the beat signals in the frequency rise region domain and the succeeding frequency fall domain. Other-wise, the distance (relative distance) and speed (relative speed) are detected from the peak frequencies of the beat signals in the frequency fall region and the succeeding frequency rise region.
In the conventional FM radar apparatus thus arranged, the distance and speed data relative to the target can be obtained at each cycle of frequency sweep of the frequency rise region and frequency fall region in combination. However, for detecting the target in more detail, it is necessary to obtain data relative to (at least one of the distance and speed of) the target at an accelerated pitch.